This invention relates to apparatus for developing a latent electrostatic image formed on the imaging surface of an electrophotographic copier, including an electrical biasing system especially intended for use therewith.
Systems that develop electrostatic latent images by applying to the imaging surface an essentially insulating carrier liquid containing suspended charged toner particles are well known in the art. In such systems, it is common to employ a developing electrode to ensure proper development of broad images and to prevent the deposition of toner particles in "background" regions that correspond to the white background of an original but which nevertheless retain some electrostatic charge.
In one type of liquid developing system, shown in Miyakawa et al. U.S. Pat. No. 4,035,071, a curved development electrode is disposed in closely spaced relation with the photoconductive imaging drum, and developer is supplied to the region of adjacency through an orifice in the electrode. The electrode is provided with a "biasing" potential greater than that of the background areas of the latent image but less than that of the areas corrresponding to the printed or typed portions of the original. A common method of biasing the development electrode is simply to allow it to float electrically, within predetermined limits, so that it assumes the average potential of the adjacent imaging surface. As a result of this biasing potential, the electrical field in the background portions of the region of adjacency is of such a polarity as to draw toner particles toward the electrode rather than toward the imaging surface, thus preventing undersirable "background" deposition.
While developing electrodes of the prior art prevent background deposition on nonimage areas, they also introduce problems of their own. Since a very high percentage of a typical original consists of background areas, the usual direction of toner particle migration in the region between the electrode and the imaging surface will be in the direction of the electrode. As a result, the developing electrode gradually accumulates a toner deposit of its own, which eventually interferes with the operation of the developing system. While it is possible to remove this deposit electrically by applying a biasing potential of opposite polarity, this necessitates an additional cleaning operation.
Further, since the development electrode must be placed relatively far from the imaging surface to provide the necessary clearance, the region of adjacency is preferably made relatively long circumferentially to allow toner particles nearer the electrode to migrate across the gap to the imaging surface. This results in a developing station of appreciable circumferential extent, thus limiting the possibilities for reducing the size of the copier.
In addition to the foregoing, if a development electrode of appreciable circumferential extent is allowed to float electrically, the level of biasing potential present when the leading portion of the latent image enters the developing region is influenced not only by the average potential of that image portion, but also by the average potential on the preceding non-image-carrying portion of the imaging surface already in the developing region. If the average potential of this preceding portion is lower than that of the leading image portion, the development electrode will be underbiased and may allow toner deposition in background areas.